A storage system generally provides a higher-level device (e.g., host) with a logical volume that is created based on a RAID (Redundant Array of Independent Disks) group configured by a plurality of storage devices. In recent years, a nonvolatile semiconductor storage device with a plurality of nonvolatile memory chips is employed as each of the storage devices, in addition to or in place of an HDD (Hard Disk Drive). For example, a flash memory device with a plurality of flash memory chips (“FM chips,” hereinafter) is employed as the nonvolatile semiconductor storage device. The technology described in Patent Literature 1, for example, is known as a storage system having such a flash memory device.